This invention relates in general to devices having an aperture of selectable diameter and relates more particularly to such a device in which this aperture is very accurately aligned with a desired optical axis. In one particularly useful application of this apparatus, this apparatus is part of an optical system that is utilized during integrated circuit fabrication to measure accurately the alignment between successive patterned layers of the integrated circuit being fabricated.
Integrated circuits typically consist of several layers of material that are patterned and interconnected in such a manner that these layers produce the desired integrated circuit. There are many different processes for producing patterned layers. However, regardless of the manner of producing each layer, in order to achieve an acceptable yield of good circuits, it is crucial that each layer be accurately aligned to all other layers so that the circuit will operate properly.
In a typical wafer fabrication process, each of these patterned layers is formed by the steps of: (a) depositing on the wafer a layer of resist; (b) exposing this layer with radiation to produce a pattern of exposed regions in this layer; (c) developing the resist to produce a contact mask; and then (d) processing the wafer through this contact mask. In these steps, the resist can, for example, be a photoresist that is exposed by light imaged through a projection reticle or can be a resist that is sensitive to incidence of an electron beam that is controlled to produce the desired pattern of exposure in the resist. In each of these cases, the resist layer is developed to produce the contact mask. The contact mask can be used, for example, during deposition or implant steps to determine where material is added to the wafer and can also be used, for example, during etching steps to determine where material is removed from the wafer.
In most integrated circuit manufacturing processes, a stepper is utilized to produce a two dimensional pattern of identical integrated circuits on a single integrated circuit wafer, thereby greatly increasing the throughput of the integrated circuit manufacturing process. This stepper accurately translates an integrated circuit wafer in each of two perpendicular directions that are parallel to a major planar surface (i.e., the top surface) of this wafer.
Unfortunately, the stepper can produce a number of different types of misalignment between successive layers. Therefore, in general, before each production run for a given layer, a test wafer 10 is produced and a registration tool is utilized to measure the alignment between these successive layers. In each of these two layers are a set of alignment marks. The registration tool checks for alignment between each alignment mark in one layer relative to its associated alignment mark in the other layer. If there is sufficiently accurate alignment between the associated registration marks in these two layers, then a process run is initiated. If there is not sufficient alignment, then the misalignment information is utilized to adjust the stepper. This process is repeated until the required degree of alignment is achieved and then the process run is initiated.
FIGS. 1A, 1B, 1C and 2 illustrate the process of measuring the registration of marks 11 in a first layer 12 with marks 14 in a second layer 15. FIGS. 1A and 1B are side and top views, respectively, of a pair of rectangular marks 11 and 14 that are supposed to be aligned laterally such that mark 14 is centered vertically over mark 11 to produce a box-in-a-box pattern as illustrated in FIG. 1B. For example, if bottom layer 12 is a layer of metallization covered by a layer 13 of polysilicon that is to be patterned, layer 15 is photoresist that has been exposed to produce an opaque region 14.
For the case of a rectangular array of nine square alignment marks, FIG. 1C illustrates seven different types of alignment and projection errors: translation, rotation, expansion, orthogonality, bow, runout and residual (i.e., those remaining alignment errors that remain after the prior six have been eliminated).
FIG. 2 is a schematic diagram of the microscope 20 utilized in a registration tool. Light from a light source 21 is collected by a condensing lens 22 to produce an optical beam 16 that is directed through a first aperture 23 in a first opaque aperture plate 24. This light beam then passes through a second aperture 25 (the "field stop") in a second opaque aperture plate 26 to a beam splitter 27 that directs a portion of this beam through an objective lens 28 onto a test wafer 10. Objective 28 directs light from this test wafer through the beam splitter to a photodetector 29.
The wafer pattern illustrated in FIGS. 1A and 1B is positioned in the center of the objective's field of view and is illuminated, producing an image as shown in FIG. 1B. This image is projected to a photodetector 29, such as a solid state camera, that produces a video grey scale for each line (such as line X in FIG. 1B) scanned. The intensity profile produced by this scan is a function of position along line X. Many horizontal scans parallel to line X are performed in the region between lines X and Y to produce an average intensity profile as illustrated in FIG. 3. For measurement purposes, a threshold value is selected that gives an accurate measurement of the actual feature, based on the intensity profile. The threshold value of 32% of saturation has been chosen for the example in FIG. 3. Since each pixel has a digitized intensity value associated with it, the actual intensity curve 31 is not smooth, but instead is actually a series of small steps. An interpolation algorithm is used that fits a curve through the plural points measured in each transition e and f, thereby providing sub-pixel resolution of the intersection points g and h.
The second aperture 25 in FIG. 2 functions as a field stop that produces a beam that just fills the field of view of the combination of optical elements 28 and 29. The first aperture 23 controls a tradeoff between resolution and contrast. A small aperture provides greater coherence of beam 16 by producing a smaller variation in pathlength of light in beam 16, but a small aperture also produces less beam intensity thereby degrading resolution of the photodetector. At this time, for registration testing of wafer patterns produced by a stepper having a maximum acceptable registration error of 150 nanometers or less, the registration tool must measure registration to an accuracy and resolution of approximately 10 nanometers. To achieve this accuracy of registration measurement using optical wavelengths of light in beam 16, this tradeoff becomes important for registration when device design geometries are approximately one-half micron. Therefore, in previous systems, to enable variation of this resolution/contrast tradeoff to achieve the required registration measurement resolution, the first aperture 23 is typically an iris diaphragm.
Unfortunately, the aperture of an iris diaphragm is sufficiently variable in size and location that this variability can seriously degrade measurement accuracy and resolution. The mechanical variability in the size and shape of the aperture of the iris diaphragm produces variability in both the area of the aperture and in the location of the center of this aperture. Electronic feedback from the photodetector to this aperture can produce reasonable control over the area of the aperture, but the position of the center of the aperture of an iris diaphragm is still sufficiently variable that this variation seriously degrades measurement accuracy and resolution.